1. Field of the Invention
Embodiments of the present invention generally relate to the formation of a composite material on a surface of a substrate. More specifically, embodiments of the invention relate to an apparatus and methods used to form composite materials that form part of an energy storage device and/or a photovoltaic device.
2. Description of the Related Art
Composite materials that may be formed using the apparatus and methods described herein include energy storage components and photovoltaic (PV) devices. Composite materials useful in energy storage components include nanotubes (and nanofibers) that are further coated to produce highly electrically conductive components, such as electrodes. Composite materials useful for forming PV devices, include intrinsic amorphous silicon (a-Si), n-type doped silicon (Si) and/or p-type doped silicon.
Carbon nanotubes and nanofibers possess many interesting and unique properties which make carbon nanotubes and nanofibers attractive for use in many potential applications, such as cold field emission, electrochemical energy storage, high-capacity hydrogen storage media, and composite material reinforcement, to name just a few. Some of the unique and interesting properties of carbon nanotubes include great strength, high electrical and thermal conductivity, large surface area-to-volume ratios, and thermal and chemical stability. The structures of carbon nanotubes and nanofibers give rise to many of their properties.
Carbon nanotubes and nanofibers are graphitic nanofilaments with diameters ranging from about 0.4 nanometers to about 500 nanometers and lengths which typically range from a few micrometers to a few millimeters. Graphitic nanofilaments may be categorized according to at least four distinct structural types, namely, tubular, herringbone, platelet, and ribbon. The term “nanotube” may be used to describe the tubular structure whereas “nanofiber” may describe the non-tubular forms.
Carbon nanotubes are generally classified as single-walled carbon nanotubes and multi-walled carbon nanotubes. FIG. 1A is a schematic view of a single-walled carbon nanotube (SWCNT). The SWCNT 100 is a graphitic nanofilament which comprises a cylindrical carbon molecule that may be conceptualized as a one-atom thick sheet of graphite called graphene rolled into a seamless graphene tube 104 of diameter “d” and filament length “L.” The graphene tube 104 forms a cylindrical wall which is parallel to the filament axis direction. One or more of the nanotube ends 102 may be capped (see FIG. 2A) by additional carbon atoms. The diameter “d” may range from about 0.4 nanometers to a few nanometers and the filament length “L” may range from a few micrometers to a few millimeters.
The rolled graphene layer or sheet of the SWCNT 100 comprises six-member hexagonal rings of carbon atoms held together by covalent sp2 bonds. These bonds combined with the tubular graphene structure impart extraordinary strength (tensile strength) and stiffness (elastic modulus) to carbon nanotubes. The SWCNT 100, for example, may have an average tensile strength of about 30 GPa and an elastic modulus of about 1 TPa compared to stainless steel which may have a tensile strength of about 1 GPa and an elastic modulus of about 0.2 TPa. Carbon nanotubes also have a fairly low density for a solid (about 1.3 g/cm3 for SWCNT's 100), and their strength-to-weight ratio is among the highest of known materials. The electrical conductivity of the SWCNT 100 may be semiconducting or metallic depending upon how the graphene sheet is rolled to form the graphene tube 104, and metallic-type carbon nanotubes can carry electrical current densities orders of magnitude larger than those carried by the best conducting metals.
FIG. 1B is a schematic view of a multi-walled carbon nanotube (MWCNT). The MWCNT 110 may be conceptualized as one or more graphene tubes 104 of filament length “L” coaxially arranged about the SWCNT 100 of diameter “d.” The graphene tubes 104 form cylindrical walls which are parallel to the filament axis direction “A” and the walls are separated from each other by an interlayer spacing 116 of about 0.34 nm which approximates the distance between graphene layers in graphite. The number of tubes (three are shown) or cylindrical walls within the MWCNT 110 may range from two to fifty, or more. An outer nanotube 112 has a filament diameter “do” which may range from a few nanometers to several hundred nanometers or more depending upon the number of walls within the MWCNT 110.
The term “carbon nanotube” is typically used to describe a nanofilament which comprises one or more graphene layers or sheets which are parallel to the filament axis and which form tubular structures. The term “carbon nanofiber,” on the other hand, typically describes a nanofilament which comprises graphene layers which may or may not be parallel to the filament axis and which do not form tubular structures, although the structures may be formed so that the nanofibers are substantially round or polygonal in cross-section. Examples of nanofiber structures include herringbone, platelet, ribbon, stacked-cone, and other carbon nanofiber structures known in the art. Some nanofibers may have a hollow core or central hole along the filament axis of each nanofiber, while other nanofibers may have solid cores. While the term “carbon nanotube” is used herein, it should be understood that this term may refer to a carbon nanotube and/or carbon nanofiber. The carbon nanotubes may have overall shapes which include but are not limited to straight, branched, twisted, spiral, and helical.
The tubular structure of carbon nanotubes have some unique properties which are not shared by carbon nanofibers. Carbon nanofibers are more closely related to graphite which consists of graphene layers held together by interlayer van der Waals forces which are much weaker than the intra-layer bonding forces within each graphene layer. The properties of carbon nanofibers are determined by the combination of the strong intra-layer bonds and the weaker interlayer bonds of the graphene structures, whereas the properties of carbon nanotubes are determined more by the strong intra-layer bonds in the tubular graphene structures. As a result, some of the properties of carbon nanofibers may be characterized as being intermediate to the properties of carbon nanotubes and graphite.
The properties of carbon nanotubes and nanofibers make their potential use in various applications desirable. The low density, high mechanical strength, electrical conductivity, and thermal conductivity of carbon nanotubes make them attractive for potential use in composite material applications. Carbon nanofibers also have fairly low densities and may be used to improve the mechanical strength and electrical conductivity of composite materials, although carbon nanofibers typically possess much less strength than carbon nanotubes.
Carbon nanotubes and nanofibers are also attractive for potential use in energy storage applications, such as electrodes for lithium-ion batteries, supercapacitors, or fuel cells. The large surface areas of carbon nanotubes and nanofibers can form large surface areas which may provide improved charge storage capabilities for electrodes. Carbon nanofibers, in particular, have many interlayer spacings through which small ions may enter and intercalate between the graphene layers, and this property makes carbon nanofibers attractive for electrode applications. It may also be desirable to deposit additional materials, such as metals, for example, onto the carbon nanotubes or nanofibers to enhance or modify various properties (e.g., electrical conductivity, strength, stiffness, thermal expansion, density) of the composite material.
Carbon nanotubes are typically formed using laser ablation, arc discharge, or chemical vapor deposition (CVD). The techniques of laser ablation and arc discharge typically use higher processing temperatures than CVD and the higher temperatures facilitate the formation of nanotubes. However, laser ablation and arc discharge form nanotubes separately (i.e., not directly on substrates) and require post-production processing (e.g., recovery, sorting, purification) of the nanotubes before they can be applied to substrates. In contrast, CVD methods allow the formation of carbon nanotubes and nanofibers directly onto substrates. Additionally, CVD methods can produce nanotubes and nanofibers at lower temperatures while providing control over the types and sizes of carbon nanotubes and nanofibers produced. Thus, CVD may provide a cost effective means for forming carbon nanotubes or nanofibers on large area substrates.
The use of various types of substrates on which the composite materials are formed may increase the range of applications for the composite materials. The substrates may include wafers, panels, sheets, webs, and fibers, for example. Thus, it is desirable to provide a cost effective means for forming carbon nanotubes and nanofibers on a substrate that that can be used to form part of an electrode used in a battery, supercapacitor, or fuel cell. Additionally, it is desirable to provide a cost effective means for the metallization of carbon nanotubes and nanofibers formed on large area substrates used in composite materials.
One method of forming a PV cell is by depositing intrinsic silicon (i-Si) and doped silicon layers on a substrate. As the need for cost effective energy sources rises, it becomes more desirable to develop methods and apparatus for forming larger PV cells. In order to do so, the ability to deposit these composite materials on ever larger substrates, also has become more desirable.
Therefore, a need exists for a cost effective method and apparatus for the formation of composite materials on various types of substrates.